1. Field of the Invention
This invention relates to low friction plastic bearings and more particularly to bearings made from engineering plastics that are filled with lubricant fibers.
2. Description of the Prior Art
Many synthetic high polymers, sometimes referred to as engineering plastics, are available for the fabrication of functional elements of machines such as bearings, gears, cams, and other load bearing and wear surfaces. These plastics are variously distinguished by a number of physical properties, including moderate to low coefficients of friction, resistance to wear and abrasion, toughness, high impact and tensile strengths, high modulus, moderate to high temperature capabilities, lightness of weight, nongalling operation and the like. Representative of these engineering plastics are the polyamides, polyacetals, polycarbonates, polysulfones, fluorocarbons, polyesters, polyolefins, polyimides, polyamide-imides and rigid polyurethanes.
Plastic bearings made from engineering plastics are often described as being self-lubricating, by which is meant that in some applications they can be used without an external source of lubrication. In point of fact, however, plastics usually do not wear well when in direct frictional engagement with metals, but they do have the ability to wet out the metal surfaces with which they are in contact to form a lubricating film that lowers friction and wear.
To improve upon the natural lubricity of plastics, it is conventional to add lubricant fillers including inorganics, e.g. molybdenum disulfide, boron nitride, antimony trioxide and vermiculite; organics, e.g. oils, fatty acids, carbon and graphite; and low friction plastics, e.g. fluorocarbons and polyolefins. The effectiveness of a lubricant filler to reduce wear is largely related to its coefficient of friction if the filler is made available at the surface of the matrix material and will wet out the bearing surface. It can be understood that if the lubricant filler is encapsulated within matrix material, the filler will not lubricate the bearing surface until the matrix material wears away to expose the lubricant filler. This can prolong the break in period and significantly reduce the useful life of the bearing.